Technical Field
The invention relates to a light source. Particularly, the invention relates to a planar light source.
Related Art
Since liquid crystal displays (LCDs) are non-self-luminous displays, a backlight module is required to provide required light to implement a display function. Along with enhancement of environmental awareness, light-emitting devices used in the backlight module are gradually changed from cold cathode fluorescent lamps (CCFLs) to more environmental friendly light-emitting diode (LED) elements. When the LED elements are applied to the backlight module, taking a side-light type backlight module as an example, the LED elements are generally disposed on a bar-shaped printed circuit board to form an LED light bar, and the LED light bar is electrically connected to a control circuit board through a flexible printed circuit (FPC).
FIG. 1 is a top view of a conventional planar light source, and FIG. 2 is a cross-sectional view of the planar light source of FIG. 1 along a section line I-I′. Referring to FIG. 1 and FIG. 2, the conventional planar light source 100 includes a light guide plate 110 and a light source device 120. The light guide plate 110 has a light exiting surface 112, a light incident surface 114 connected to the light exiting surface 112, and a bottom surface 116 opposite to the light exiting surface 112, wherein the light exiting surface 112 includes a peripheral area 112a connected to the light incident surface 114 and a active illumination area 112b. A plurality of printed dots 116a are distributed on the bottom surface 116 of the light guide plate 110, and light emitting uniformity of the light exiting surface 112 could be adjusted by adjusting a distribution density of the printed dots 116a. The light source device 120 is disposed beside the light incident surface 114, and the light source device 120 includes a circuit board 122 and a plurality of LED elements 124, wherein the LED elements 124 are disposed on the circuit board 122 and are electrically connected to the circuit board 122. A light beam L emitted from the LED element 124 enters the light guide plate 110 through the light incident surface 114, and is propagated in the light guide plate 110, wherein a part of the light beam L is scattered by the printed dots 116a on the bottom surface 116 to leave the light guide plate 110 through the light exiting surface 112.
As shown in FIG. 1, in the light source device 120, a pitch between any two adjacent LED elements 124 is P, and a shortest distance between a light emitting surface of each LED element 124 and an edge of the active illumination area 112b is A. In order to uniform a brightness of the active illumination area 112b, the manufacturer determines an optimal A/P ratio according to a beam divergence angle α of the LED element 124. The higher the A/P ratio is, the better the light uniformity is. However, the LCDs are gradually developed towards a trend of slim border, to cope with the design requirement of slim border, the shortest distance A has to be decreased, and when the A/P ratio is excessively low, the active illumination area 112b close to the light incident surface 114 may have bright-dark alternated hot spots, and shadows in FIG. 1 are areas with lower brightness. The hot spots could be eliminated by decreasing the pitch P. However, when the pitch P is decreased, the manufacturer has to use more number of the LED elements 124, which increases the cost.
Therefore, it is important for technicians of the field to resolve the aforementioned hot spot issue under premises of not increasing the manufacturing cost and complying with the design requirement of slim border.
Moreover, Taiwan Patent No. TWI262458 and U.S. Pat. No. 7,364,343B2 disclose that the light source is disposed at a thinner side of a wedge-shaped translucent material light emitting panel. China Patent Publication No. CN102449509A discloses that a side-mounted light source is disposed at a thinner side of a wedge-shaped light guide, and a thicker side of the wedge-shaped light guide has a reflective coating. China Patent Publication No. CN102483522A and U.S. Pat. No. 8,189,973B2 disclose that the light source is disposed at a thinner end of an optical wedge, and a thicker end of the optical wedge has a reflector, and the reflector is bended with a radius of curvature having a center of curvature. U.S. Pat. No. 7,360,939B2 discloses that a light beam emitted from a front light source (light guide plate) passes through an LCD, and is reflected by a reflective film and passes through the LCD, wherein the light guide plate has a plurality of first grooves and second grooves. U.S. Pat. No. 7,654,722B2 discloses that a plurality of V-shaped reflective structures are configured on a reflective surface of a light guide plate. Taiwan Patent No. TWI356204 discloses that two oval light incident structures of different sizes are formed on a light incident surface of a light guide plate. U.S. Pat. No. 6,254,245B1 discloses that a side light source is disposed at a thinner side of a light guide plate. China Patent Publication No. CN102349006A discloses that a plurality of light guide portions are disposed at multiple segments of light guides and have a plurality of reflectors. Taiwan Patent Publication No. TW201020638 discloses that light beams sent by light-emitting components are emitted in parallel, and the light beams are propagated to a curved surface of a reflective device and are incident to a light incident surface of the light guide plate after at least one reflection.